In high snow load regions, roof top snow can melt into water due to escaping heat from the structure's interior or thermal radiation from the sun. When this water drains to the colder overhang or other colder roof surfaces the water can refreeze creating an ice dam. Ice dams can also form in the valley between adjoining roof surfaces or next to roof protrusions such as chimneys, dormers or second story structures. A continuously heated drain path, including heated gutters and down spouts, ensure the water is drained away from the structure's foundation.
These ice dams can prevent additional snow melt water drainage and standing water is formed above the ice dam. Most slanted roofs are designed to shed moving water like an umbrella, not to hold standing water like a swimming pool. This standing water can penetrate a standard roof and enter the structure causing interior damage, mold, mildew and electrical issues. If this water is allowed to re-freeze, the change-of-state expansion can cause major structural damage.
To provide a heated drain path though the ice dam, the state of the art for professional installations utilizes self-regulating electrical heating cable that increases the heat output per foot when the cable is in direct contact with ice and snow. When the drainage path around the cable is established, the air pocket acts like a storm window and greatly reduces the thermal conduction between the heating cable and the ice and snow. As the electrically semi-conductive cable core heats up, the electrical resistance between the electrical bus wires increases and the electrical wattage used per foot decreases. A typical self-regulating ice and snow melt cable can create a load of 24 watts per foot in ice and snow at −10° F. but drops to about 6 watts per foot in ambient air at 32° F. So the air pocket around the cable becomes an important variable in the system and product design.
Increasing the size of the melt path has been the goal of the industry for many years. A single run of cable provides about a two inch wide drain path. A zig-zag placement of the cable provides a wider coverage area but ice can form between the zig and the zag and the additional cable length adds cost and requires more electrical power.
Embedding the cable in a fixed width cavity inside a thermally conductive metal panel system works quite well to increase the size of the melt path, which is the goal. Some heavy weight extruded panels are offered that have a high fixed thermal mass, ensuring maximum heat sinking and electrical power usage from the self-regulating heating cable. However, over time the thermal expansion and contraction of the cavity size of any current panel design reduces the contact area between the cable and the metal. A very small thermal air gap can reduce the self-regulating cable's heat output and the temperature of the metal panel dramatically.